MC34025, MC33025
PIN FUNCTION DESCRIPTION
Pin No.
DIP/SOIC
1
2
3
4
5
6
7
8
9
10
11
12
13
Function
Error Amp Inverting
Input
Error Amp
Noninverting Input
Error Amp Output
Clock
R
T
C
T
Ramp Input
Soft−Start
Current
Limit/Shutdown
Ground
Output A
Power Ground
V
C
Description
This pin is usually used for feedback from the output of the power supply.
This pin is used to provide a reference in which an error signal can be produced on the output of the
error amp. Usually this is connected to V
ref
, however an external reference can also be used.
This pin is provided for compensating the error amp for poles and zeros encountered in the power
supply system, mostly the output LC filter.
This is a bidirectional pin used for synchronization.
The value of R
T
sets the charge current through timing Capacitor, C
T
.
In conjunction with R
T
, the timing Capacitor sets the switching frequency. Because this part is a
push−pull output, each output runs at one−half the frequency set at this pin.
For voltage mode operation this pin is connected to C
T
. For current mode operation this pin is
connected through a filter to the current sensing element.
A capacitor at this pin sets the Soft−Start time.
This pin has two functions. First, it provides cycle−by−cycle current limiting. Second, if the current is
excessive, this pin will reinitiate a Soft−Start cycle.
This pin is the ground for the control circuitry.
This is a high current totem pole output.
This is a separate power ground return that is connected back to the power source. It is used to
reduce the effects of switching transient noise on the control circuitry.
This is a separate power source connection for the outputs that is connected back to the power
source input. With a separate power source connection, it can reduce the effects of switching
transient noise on the control circuitry.
This is a high current totem pole output.
This pin is the positive supply of the control IC.
This is a 5.1 V reference. It is usually connected to the noninverting input of the error amplifier.
14
15
16
Output B
V
CC
V
ref
4
5
Oscillator
C
T
6
C
T
4
5
Oscillator
6
7
3
1
Output Voltage
Feedback Input
V
ref
2
1.25 V
From Current
Sense Element
7
3
1
1.25 V
Output Voltage
Feedback Input
V
ref
2
In voltage mode operation, the control range on the output of
the Error Amplifier from 0% to 90% duty cycle is from 2.25 V
to 4.05 V.
In current mode control, an RC filter should be placed at the
ramp input to filter the leading edge spike caused by turn−on of
a power MOSFET.
Figure 22. Voltage Mode Operation
Figure 23. Current Mode Operation
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